Origin

The word deep is related to dip (Old English) and dive (Old English), and in Old English could also mean depth (Late Middle English). The phrase in deep water, ‘in trouble or difficulty’, has biblical origins. The writer of one of the Psalms begged, ‘Let me be delivered from them that hate me, and out of the deep waters’. The deep waters of a swimming pool did not become familiar enough to provide linguistic inspiration until the 20th century. If you go off the deep end you have an emotional outburst, especially of anger, and to jump (or be thrown) in at the deep end is to face a difficult undertaking with little or no preparation or experience.